System of converting information words to a signal

ABSTRACT

A method of converting m-bit information words to a runlength constrained modulated signal is described. The available code words are distributed over at least one group (G 1 ) of a first type and at least one group (G 2 ) of a second type. The delivery of a code word belonging to the group of the first or second type establishes a coding state of the first type (S 1 ) or one of a number r of coding states (S 2,  S 3 ) of the second type depending on the current information word. For each information word a subset of code words is available, which subset has at least one disjunct code word for each of the r coding states. The selection from the subset of the code word to be delivered is based the coding state, on dynamically verifying the runlength constraint for the sequence of code words and on an additional criterion, like the low frequency content of the modulated signal.

The invention relates to a method of converting a series of m-bitinformation words to a modulated signal, with m being an integer, inwhich method an n-bit code word is delivered for each receivedinformation word, with n being an integer exceeding m, and the deliveredcode words are modulated to the modulated signal, and in which theseries of information words is converted to a series of code words sothat the corresponding modulated signal satisfies a predeterminedcriterion, and in which the code words are spread over at least a groupof a first type and at least a group of a second type, while thedelivery of a code word belonging to the group of the first typeestablishes a coding state of a first type determined by the associatedgroup, and the delivery of a code word belonging to the group of thesecond type establishes one of r coding states of a second typedetermined by the associated group ánd by the information wordassociated to the delivered code word, in which the group of the secondtype comprises at least one code word being associated with at most rinformation words among which the respective information word isdistinguishable by evaluating a predetermined parameter of the followingcode word.

The invention further relates to a method for producing a record carrieron which a signal is recorded obtained according to said method.

The invention further relates to a coding device comprising an m-to-nbit converter for converting the m-bit information words to n-bit codewords, and means for modulating the n-bit code words to a modulatedsignal, the modulated signal satisfying a predetermined criterion, andstate establishing means for establishing, on the delivery of a codeword by the converter, a coding state of a first type for each of thedelivered code words belonging to a group of a first type determined bythe associated group, and one of r coding states of a second type foreach of the delivered code words belonging to a group of the second typedetermined by the associated group ánd by the information wordassociated to the delivered code word, in which the group of the secondtype comprises at least one code word being associated with at most rinformation words among which the respective information word isdistinguishable by evaluating a predetermined parameter of the followingcode word.

The invention further relates to a signal comprising a sequence ofsuccessive signal portions each corresponding to an information word,the signal satisfying a predetermined criterion, in which signal each ofthe signal portions comprises n bit cells having a first or secondsignal value, a signal portion belonging to a group of a first type ofsignal portions uniquely representing an information word, and a signalportion belonging to a group of a second type of signal portions incombination with a successive signal portion representing a uniqueinformation word, at least one signal portion of the group of the secondtype being associated with at most r information words among which therespective information word is distinguishable by evaluating apredetermined parameter of the successive signal portion.

The invention further relates to a record carrier on which the signal isrecorded.

The invention further relates to a rendering device comprising means forreading information from a track on a record carrier, which devicecomprises means for scanning the track and demodulation means forretrieving code words from signal portions of a signal provided in thetrack, the signal satisfying a predetermined criterion, in which signaleach of the signal portions comprises n bit cells having a first orsecond signal value, a signal portion belonging to a group of a firsttype of signal portions uniquely representing an information word, and asignal portion belonging to a group of a second type of signal portionsin combination with a successive signal portion representing a uniqueinformation word, and a converter for converting the code words intoinformation words, the converter comprising means for distinguishing therespective information word by evaluating a predetermined parameter ofthe following code word.

Such methods, such devices, such a record carrier and such a signal areknown from U.S. Pat. No. 5,790,056. The document relates to a method ofconverting a series of m-bit information words to a modulated signal,the method being called EFM+ and being used for encoding data on opticaldiscs like DVD. For each information word from the series an n-bit codeword is delivered. The delivered code words are converted to a modulatedsignal. The code words are distributed over at least one group of afirst type and at least one group of a second type. For the delivery ofeach of the code words belonging to the group of the first type theassociated group establishes a coding state of the first type. When acode word belonging to the group of the second type is delivered, acoding state of the second type is established. A code word is assignedto the received information word selected from a set of code words whichdepends on the established coding state. The sets of code wordsbelonging to the coding states of the second type are disjunct. Theselected one of the possible coding sets of the second type isdetermined by the information word associated to the delivered codeword. This allows several information words being associated with thesame code word, the established coding state being different. In thiscoding method the number of unique bit combinations that may be used bythe code words in the series is enlarged, thereby increasing the codingefficiency. The modulated signal thus obtained, stored and/ortransmitted can be reconverted to information words by first convertingthe modulated signal to a series of code words and then assigning aninformation word to each of the code words from the series in dependenceon the code word to be converted and also in dependence on a parameterof the following code word, e.g. values of bits which are situated atpredetermined positions relative to the current code word, which logicalvalues are indicative for the previously established coding state.

Low frequency components of the modulated signal should be constrainedto a low level, in particular DC is to be avoided. The main reason isthat low frequency components may interfere with other signals, such asservo tracking signals for controlling the position of the optical headin a direction transverse to the track. To make DC control possible, thedescribed coding system has partial double tables with opposite DCcontribution and allows further selection of an alternative code word inthe event that a code word from a coding set of the first type isselected, provided that the predetermined criterion of the modulatedsignal, i.e. the runlength constraints for minimum and maximum runlengthof bit cells having the same signal value, is not violated. Although thedescribed converting method results in a modulated signal with limitedlow frequency content, there still is a need to decrease the lowfrequency components or to otherwise control the properties of themodulated signal.

Therefore it is an object of the invention to provide means forconverting adapted for improved control of the properties of themodulated signal.

According to a first aspect of the invention this object is achievedwith a method as in the opening paragraph, characterized in that eachinformation word is provided with an associated subset of code wordscomprising at least r code words mutually differing in the value of thepredetermined parameter, and

the delivered code word is selected based on an additional criterionfrom the subset of code words excluding all code words not complyingwith said predetermined criterion and, in the event that a coding stateof the second type has been established by the preceding code word,excluding all code words having a value of the predetermined parameternot corresponding to the established coding state of the second type.

According to a second aspect of the invention this object is achievedwith a coding device as in the opening paragraph, characterized in thatthe m-to-n-bit converter comprises means for providing for eachinformation word an associated subset of code words comprising at leastr second type of code words mutually differing in a predetermined way,and means for selecting a code word based on an additional criterionfrom the subset of code words excluding all code words not complyingwith said predetermined criterion and, in the event that a coding stateof the second type has been established by the preceding code word,excluding all code words having a value of the predetermined parameternot corresponding to the established coding state of the second type.

According to a third aspect of the invention this object is achievedwith a signal as in the opening paragraph, characterized in that thesignal comprises at least one signal portion selected based on anadditional criterion from a subset of signal portions excluding allsignal portions not complying with said predetermined criterion and, inthe event that the preceding signal portion belongs to the group of thesecond type, excluding all signal portions having a value of thepredetermined parameter not being associated with the respectiveinformation word, and the subset being associated to an information wordand comprising at least r signal portions mutually differing in thevalue of the predetermined parameter.

According to a fourth aspect of the invention this object is achievedwith a rendering device as in the opening paragraph, characterized inthat the converter comprises means for converting a code word selectedfrom a subset of code words, the subset being associated to theinformation word and comprising at least r code words mutually differingin the value of the predetermined parameter.

The invention is based on the following recognition. Conventionally thecode words are generated using a finite state machine approach, and thecode words for each coding state are defined in a set, e.g. like acolumn in a memory table. When a code word must be selected according tothe established coding state, the entry in the table corresponding tothe row of the information word and the column of the coding state isaccessed. The compliance with the predetermined criterion, i.e. therunlength constraints of the signal, is guaranteed by strict conditionson the code words in each column. The inventor has seen that although inthe known coding system always a code word is available, this is notnecessarily the best choice. Hence now code words are no longerconsidered to be part of a set corresponding to a state. For eachinformation word a subset of code words is available from which a bestchoice is made as follows. First the runlength criterion is dynamicallyevaluated by excluding all code words not complying with the runlengthconstraints, and secondly, in the event of a second type coding state,all code words are excluded which do not have the required value of theparameter corresponding to the actual second type coding state. Finally,if more than one code word of the subset complies with the criteria justnamed, a further choice can be made based on an additional criterion,e.g. the low frequency content of the signal. Surprisingly a furtherimprovement of the low frequency suppression of about 3 dB has beenfound when comparing the modulated signal according to the inventionwith the traditional DVD EFM+ coding system as described above.

Further preferred embodiments of the method, devices and record carrieraccording to the invention are given in the further claims.

These and other aspects of the invention will be apparent from andelucidated further with reference to the embodiments described by way ofexample in the following description and with reference to theaccompanying drawings, in which

FIG. 1 shows a series of information words, a corresponding series ofcode words and a modulated signal;

FIG. 2 shows a record carrier;

FIG. 3 shows an enlarged portion of the record carrier of FIG. 2;

FIG. 4 shows a recording device;

FIG. 5 shows a decoding and playback device.

FIG. 6 shows a coding device;

FIG. 7 a-i shows tables which have subsets of code words for each of theinformation words; and

FIG. 8 a-b shows the first part of the tables for conventional EFM+encoding.

In the Figures, elements which correspond to elements already describedhave the same reference numerals.

FIG. 1 shows three consecutive m-bit information words, in this case,8-bit information words referenced 1. Information about coding methodscan be found in the book by K. A. S. Immink, Codes for Mass Data StorageSystems, Shannon Foundation Publishers, Eindhoven, The Netherlands, 1999(ISBN 90-74249-23-X) and the article by K. A. S. Immink, ‘A Survey ofCodes for Optical Disc Recording’, IEEE Journal on Selected Areas ofCommunications, Special issue on signal processing and coding fordigital storage, vol. 19, pp. 751-764, April 2001. In said title, forexample, the so-called EFM modulation system is described which is usedfor recording information on so-called Compact Discs and the EFM+ systemused in DVD. The EFM-modulated signal is obtained by converting a seriesof 8-bit information words to a series of 14-bit code words, threemerging bits being inserted into the code words. The code words areselected such that the minimum number of “0” bits situated between the“1” bits is d (2) and the maximum number is k (10). This constraint isalso referenced dk-constraint. The series of code words is converted,via a modulo-2 integration operation, to a corresponding signal formedby bit cells having a high or low signal value, a “1”-bit beingrepresented in the modulated signal by a change from the high to the lowsignal value or vice versa. A “0”-bit is represented by the lack of achange of signal value at a transition between two bit cells. Themerging bits are selected such that even in the regions of transitionbetween two code words the dk-constraint is satisfied and that in thecorresponding signal the so-called running digital sum value remainssubstantially constant. The running digital sum value at a specificinstant is understood to mean the difference between the number of bitcells having the high signal value and the number of bit cells havingthe low signal value, calculated over the modulated signal portionsituated before this instant. A substantially constant running digitalsum value means that the frequency spectrum of the signal does notcomprise frequency components in the low frequency area. Such a signalis also referenced a DC-free signal. The lack of low-frequencycomponents in the signal is highly advantageous when the signal is readfrom a record carrier on which the signal is recorded in the track,because then continuous tracking control unaffected by the recordedsignal is possible. Information recording has a constant need forenhancing the information density on the record carrier. In FIG. 1 thethree information words 1 have the respective word values “34”, “138”and “34”. This series of 3 information words 1 is converted to threeconsecutive n-bit code words, in this case, 16-bit code words referenced4. The code words 4 form a bit string of bits having a logical “0” valueand bits having a logical “1” value. The conversion of the informationwords is such that in the bit string the minimum number of bits having alogical “0” value positioned between two bits having a logical “1” valueis d and the maximum is k, where d is equal to 2 and k is equal to 10.Such a bit string is often referenced a RLL string (RLL=Run LengthLimited) with a dk-constraint. The individual bits of the code wordswill further be referenced x1, . . . , x16, where x1 denotes the firstbit from the left, usually called the most significant bit, of the codeword and x16 denotes the last bit of the code word.

The bit string formed by the code words 4 is converted to a modulatedsignal 7 by means of a modulo-2 integration operation. This modulatedsignal comprises three information signal portions 8 representing thecode words 4. The information signal portions comprise bit cells 11which may have a high signal value H or a low signal value L. The numberof bit cells per information signal portion is equal to the number ofbits of the associated code word. Each code word bit having a logical“1” value is indicated in the modulated signal 7 by a transition from abit cell having the high signal value to a bit cell having the lowsignal value, or vice versa. Each code word bit having the logical “0”value is indicated in the modulated signal 7 by the absence of a changeof signal value at a bit cell transition.

Furthermore, the frequency spectrum of the modulated signal 7 isrequired to include substantially no low-frequency components. Wordeddifferently, the modulated signal 7 is to be substantially DC-free.

In the following an embodiment of the method according to the inventionby which the modulated signal can be obtained will be described indetail.

The total set of code words is established as follows. First there is arequirement with respect to the code words that within the code wordsthe dk-constraint is satisfied. The set of all the possible code wordssatisfying said dk-constraint is then divided into at least a group of afirst type and at least a group of a second type. When a code word isdelivered from one of the groups of the first type, a coding state S1 isestablished which exclusively depends on the group of the first type towhich the delivered code word belongs. When one of the code words of thegroup of the second type is delivered, a coding state is establishedwhich depends both on the group of the second type and on theinformation word represented by the delivered code word. In enembodiment the group of code words of the first type referred to as G1includes code words ending in a number a of bits having a logical “0”value, where the number a may be an integer equal to 0 or 1, or aninteger smaller than or equal to 9 and greater than or equal to 6. Theembodiment described here only has one group of the second type. Thisgroup comprises code words ending in a different number b of bits havinga logical “0” value, which number b is an integer greater than or equalto 2 and smaller than or equal to 5. This group will henceforth bereferenced group G2. In the example to be described here, two codingstates i.e. S2 and S3 can be established by the combination of a codeword and associated information word.

For each information word a subset of code words is established suchthat for each of the number of ending “0” bits a next code word isavailable so that the resulting sequence complies with the runlengthconstraint. Hence each subset has at least one code word having 0 or 1zeroes at the start, and also at least one code word having at least 2zeroes at the start. Further the subset has at least one code word forstate S2 and at least one for state S3, which two code words aredistinguishable based on a parameter which can be derived from the codeword, e.g. the values of two bit positions both being zero or at leastone being one. Hence for each information word at least one code wordfor S2 and at least one code word for S3 are available. Additional codewords can be added to subsets, which can be used to comply with anadditional criterion, e.g. the low frequency content of the signal. Inan embodiment the additional code words are added for the subsets havingcode words with a high DC content, the added code words having anopposite DC content. In such case the low frequency content can bereduced by making a suitable selection from the subset of code wordsusing the all code words which comply with the runlength constraint,i.e. for which the modulated signal of concatenated code words has onlyrunlengths within the required dk range. In an embodiment the selectionof code words from the subsets is used to encode further information ofa low density, e.g. a watermark or copyright information. Such encodingmay be also be combined with a reduction of the low frequency componentsin the modulated signal, in which case the reduction obviously will besomewhat less than in the case where the additional criterion is lowfrequency content only.

It is noted that dynamically choosing a code word that complies with theparameter value required by the state (needed only for a state of thesecond type) and with the dk constraint, i.e. using any of the codewords from the subset that comply with the state and the runlengthrequirements, is more flexible than the conventional coding system basedon a state machine approach, e.g. used in the EFM+ code for DVD, asdescribed with reference to FIG. 8. The additional reduction of the LFcomponents of the dynamic choosing compared to EFM+ is about 3 dB.

FIG. 2 shows by way of example, a record carrier 120 according to theinvention. The record carrier shown is one of an optically detectabletype. The record carrier may also be of a different type, for example,of a magnetically readable type. The record carrier comprisesinformation patterns arranged in tracks 121.

FIG. 3 shows a strongly enlarged portion 122 of one of the tracks 121.The information pattern in the track portion 121 shown in FIG. 3comprises first sections 123, for example, in the form of opticallydetectable marks and second sections 124, for example, intermediateareas lying between the marks. The first and second sections alternatein a direction of the track 125. The first sections 123 present firstdetectable properties and the second sections 124 present secondproperties which are distinguishable from the first detectableproperties. The first sections 123 represent bit cells 12 of themodulated binary signal 7 having one signal level, for example, the lowsignal level L. The second sections 124 represent bit cells 11 havingthe other signal level, for example, the high signal level H. The recordcarrier 12 may be obtained by first generating the modulated signal andthen providing the record carrier with the information pattern. If therecord carrier is of an optically detectable type, the record carriercan then be obtained with mastering and replica techniques known per sebased on the modulated signal 7.

FIG. 4 shows a recording device for recording information, in which thecoding device according to the invention is used, for example, thecoding device 140 shown in FIG. 6. In the recording device the signalline for delivering the modulated signal is connected to a controlcircuit 141 for a write head 142 along which a record carrier 143 of awritable type is moved. The write head 142 is of a customary typecapable of introducing marks having detectable changes on the recordcarrier 143. The control circuit 141 may also be of a customary typegenerating a control signal for the write head in response to themodulated signal applied to the control circuit 141, so that the writehead 142 introduces a pattern of marks that corresponds to the modulatedsignal.

FIG. 5 shows a reading device in which a decoding device according tothe invention is used, for example, a decoding device 153 as describedbelow. The reading device comprises a read head of a customary type forreading a record carrier according to the invention which record carriercarries an information pattern that corresponds to the modulated signal.The read head 150 then produces an analogue read signal modulatedaccording to the information pattern read out by the read head 150.Detection circuit 152 converts this read signal in customary fashion toa binary signal which is applied to the decoding circuit 153.

An embodiment of the decoding device 153 consists of a logic array thatimplements the inverse of the coding function. The decoding device 153includes a ROM memory as decoder, and/or a gate circuit for storing thesubsets of code words for each information word as shown in FIG. 7. Thereceived code words are compared with the stored code words, and theinformation word corresponding to the subset containing the receivedcode word is send to the output. In the event that a code word is partof two subsets, the respective subset is selected by observing the 1stand 13th bit of the upcoming code word, which indicate the coding stateof that upcoming code word of the second type S2 or S3. For each codeword in the subsets a next coding state value is also stored, asindicated in FIG. 7. The applicable subset is determined by comparingthe detected coding state with the stored states.

FIG. 6 shows an embodiment for a coding device 140 according to theinvention by which the coding described above can be carried out. Thecoding device is arranged for converting the m-bit information words 1to the n-bit code words 4 and the number of different coding states canbe indicated by s bits. The coding device comprises a converter 60 forconverting (m+s+x) binary input signals to (n+s+t) binary outputsignals. From the inputs of the converter m inputs are connected to abus 61 for receiving m-bit information words. From the outputs of theconverter n outputs are connected to a bus 62 for delivering n-bit codewords. Furthermore, s inputs are connected to an s-bit bus 63 forreceiving a state word denoting the current coding state. According tothe embodiment of the coding described above the coding state can havethe value S1, S2 or S3. A state word is delivered via the s-bit bus 63by a buffer memory 64, for example, in the form of s flip-flops. Thebuffer memory 64 has s inputs connected to a bus 58 for receiving astate word to be stored in the buffer memory. For delivering the statewords to be stored in the buffer memory the converter 60 has s outputsthat are connected to bus 58. The converter 60 may comprise a ROM memoryin which the code word tables shown in FIG. 7 are stored. The addressesin the ROM are determined by the combination of state word andinformation word applied to the inputs of the converter, while also inresponse to a selection signal 77, the addresses of the memory locationsare selected. In a different embodiment the state words are derived by agate circuit.

In a first operation the converter 60 selects a subset of code wordscorresponding to the present information word on bus 61. Secondly wordsof the subset are selected that comply with the state on s-bit bus 63and the predefined criterion, e.g. the runlength constraint dk, for themodulated signal as follows. If the coding state is of the first type,all code words of the subset are selected, but if the coding sate is ofthe second type, like S2 or S3, only code words complying with therequired parameter, e.g. the bit values at positions x1 and x13, forthat state are selected. Further only code words complying with therunlength constraints are selected by calculating the runlengthsoccurring in the sequence of code words at the boundary between thepreceding and the current code word. After the selection one or morecode words are available for encoding the present information word. Theconverter applies to a bus 75 information of all selected code words inthe subset. The bus 75 is connected to a selection circuit 76 forapplying an additional criterion to the selection of the code word fromthe subset. In an embodiment the parameter is the low frequency content.The parameter evaluated in selection circuit 76 then denotes for each ofthe selected code words the change of the digital sum value caused bythat code word. The selection circuit 76 calculates a running DSV for aportion of the modulated signal. This portion may start at an arbitrarypoint in the past or at a sync word. In another embodiment the DSV mayalso be calculated for a future portion, but in that case a memory isneeded for temporarily storing the possible sequences of code words.Based on this information the selection circuit 76 delivers a selectionsignal of x bits which indicates which one of the code words selectedfrom the subset of the current information word is to be fed to the bus62. This selection signal is applied to the converter 60 over aselection signal line 77.

It is noted that a sync pattern can be added to the modulated signalregularly. The definition of a unique and reliable pattern can be thesync pattern as used in the DVD system, or the sync patterns in thecoding system described in U.S. Pat. No. 5,696,505.

Finally bus 62 is connected to the parallel inputs of aparallel-to-serial converter 66 which converts code words 4 receivedover bus 62 to a serial bit string to be supplied over a signal line 67to a modulator circuit 68 which converts the bit string to the modulatedsignal 7 to be delivered over signal line 70. The modulator circuit 68may be one of a customary type, for example, a so-termed modulo-2integrator.

In an embodiment the decoding device 153 as described with FIG. 5includes a demodulator for retrieving information encoded in theselection of the code words. The demodulator comprises a selectioncircuit similar to the selection circuit 76 in the coding device 140.The selection circuit reconstructs a sequence of code words based on theinformation words retrieved. The demodulator compares the sequence ofcode words as received and the reconstructed sequence. From thedifferences additional information is decoded. The additionalinformation may be a watermark or copyright information.

FIG. 7 shows a coding table according to the invention; subdivided inparts in FIGS. 7 a-7 i. In the first column marked “Data” theinformation words 0-255 are listed. In each data row in the even columnsup to 5 possible code words are listed, constituting a subset of codewords for the information word in that row. The second, sixth and tenthcolumns are marked “S2”, and contain code words usable when the codingstate is S2. The code words comply with the parameter value for thisstate, i.e. having a zero value on bit positions x1 and x13. The fourth,eighth and twelfth columns are marked “S3” and contain code words usablewhen the coding state is S3. The code words comply with the parametervalue for this state, i.e. having a non-zero value on bit positions x1or x13. When the coding state is S1 code words from all columns can beused. In the odd columns marked “NS” the next state is indicated for thecorresponding code word given left of that value. A next state 2indicates that the following code word must be taken from a columnmarked S2, and next state 3 indicates that the following code word mustbe taken from a column marked S3. A next state x indicates the codingstate of the first type S1; and x indicates that the following code wordmay be taken from any of the columns. As explained above the sequence ofcode words must also comply with a predefined criterion, i.e. runlengthconstraints of a minimum number of zeroes d=2 and a maximum number ofzeroes k=10. Hence only code words are selected that when combinedconstitute a sequence having such only allowed runlengths. Further thefinal selection of the code word is based on an additional criterion,i.e. reducing the low frequency content or a further modulation of lowfrequency components.

FIG. 8 shows part of a coding table for the conventional EFM+ code, asfully described in U.S. Pat. No. 5,790,056. The rate is 8/16, so m=8 andn=16, the dk constraints are d=2, k=10. A main and a substitute tableare used, and FIG. 8 a shows the first part of the main coding table andFIG. 8 b shows a first part of the substitute table for the informationwords 0-87. The code words are selected based on a conventional statemachine approach as described below. The code words of the substitutetable may be selected for DC control. When the information words areconverted to code words, a code word belonging to a set of code wordsdepending on one of the coding states S1,S2, DS3 or See information wordto be converted. The sets of code words belonging to the coding statesS1, S2, S3 and S4 will henceforth be referenced V1, V2, V3 and V4,respectively. The code words in the sets are selected such that each bitstring that can be formed by a code word from the group that hasestablished a coding state and an arbitrary code word from the setestablished by this coding state satisfies the dk-constraint. In thecase where the coding state S4 is established by the delivery of thepreviously delivered code word and the coding state thus denotes thatthe previous code word ends in a bit string having a logical “0” valuegreater than or equal to 6 and smaller than or equal to 9, code word setV4 which is established by the coding state S4 is only allowed tocomprise code words beginning with a maximum of 1 bit having the logical“0” value. It is noted that code words beginning with a larger number ofbits having the logical “0” value would result in transitional areasbetween the previously delivered code word and the code word to bedelivered, in which areas the number of successive bits having thelogical “0” value will not always be smaller than or equal to 10 andthus not satisfy the dk-constraint. For similar reasons, set V1comprises only code words beginning with a number of bits having thelogical “0” value that is greater than or equal to 2 and smaller than orequal to 9.

Sets V2 and V3 of code words belonging to the coding states S2 and S3contain only code words beginning with a number of bits having a logical“0” value greater than or equal to 0 and smaller than or equal to 5. Thecode words satisfying this condition are spread over the two sets V2 andV3, so that sets V2 and V3 do not contain any common code words at all.Sets V2 and V3 will be referenced disjunct sets in the following. Thespreading of the code words over sets V2 and V3 is preferably such thaton the basis of the logical values of a limited number of p bits therecan be determined to what set a code word belong. In the exampledescribed above, the bit combination x1.x13 is used for this purpose.Code words from set V2 are recognisable from the bit combinationx1.x13=0.0. Code words from set V3 are then recognisable from thecombination x1.x13 which is unequal to 0.0. A distinction is madebetween code words establishing coding state S1 (group G11) on delivery,code words establishing coding state S2 or S3 (group G2) on delivery,and code words establishing the coding state S4 (group G12) on delivery.Set V1 comprises 138 code words from group G11, 96 code words from groupG2 and 22 code words from group G12. It will be evident that the numberof different code words in set V1 is smaller than the number ofdifferent 8-bit information words.

Since the code words from group G2 are always followed by a code wordfrom set V2 or a code word from set V3, and, in addition, based on thecode word following a code word from group G2 there may be establishedwhat set this code word belongs to, a code word from group G2 followedby a code word from set V2 can be unequivocally distinguished from thesame code word from group G2, but followed by a code word from set V3.Worded differently, when code words are assigned to an information word,each code word from group G2 can be used twice. Each code word fromgroup G2 together with a random code word from set V2 forms a unique bitcombination which is inseparable from the bit combination formed by thesame code word and a random code word from the same set V3. This meansthat 138 unique bit combinations (code words) from group G11 can be usedfor set V1, 22 unique bit combinations (code words) from group G12 and2*96 unique bit combinations (code words from group G2 combined withsubsequent code words) from group G2. This brings the total number ofuseful unique bit combinations to 352. The number of unique bitcombinations formed with the code words from sets V2, V3 and V4 are 352,351 and 415, respectively.

Although the invention has been explained by embodiments using codewords from the EFM+ code, any other suitable set of code words andrunlength constraints may be used. Also for the record carrier anoptical disc has been described, but other media, such as a magneticdisc or tape, may be used. It is noted, that in this document the word‘comprising’ does not exclude the presence of other elements or stepsthan those listed and the word ‘a’ or ‘an’ preceding an element does notexclude the presence of a plurality of such elements, that any referencesigns do not limit the scope of the claims, that the invention may beimplemented by means of both hardware and software, and that several‘means’ may be represented by the same item of hardware. Further, thescope of the invention is not limited to the embodiments, and theinvention lies in each and every novel feature or combination offeatures described above.

1. Method of converting a series of m-bit information words (1) to amodulated signal (7), with m being an integer, in which method an n-bitcode word (4) is delivered for each received information word (1), withn being an integer exceeding m, and the delivered code words (4) aremodulated to the modulated signal (7), and in which the series ofinformation words is converted to a series of code words so that thecorresponding modulated signal (7) satisfies a predetermined criterion,and in which the code words (4) are spread over at least a group of afirst type (G11,G12) and at least a group of a second type (G2), whilethe delivery of a code word belonging to the group of the first type(G11,G12) establishes a coding state (S1) of a first type determined bythe associated group, and the delivery of a code word belonging to thegroup of the second type (G2) establishes one of r coding states (S2,S3)of a second type determined by the associated group and by theinformation word (1) associated to the delivered code word (4), in whichthe group of the second type comprises at least one code word beingassociated with at most r information words among which the respectiveinformation word is distinguishable by evaluating a predeterminedparameter of the following code word, characterized in that eachinformation word is provided with an associated subset of code wordscomprising at least r code words mutually differing in the value of thepredetermined parameter, and the delivered code word is selected basedon an additional criterion from the subset of code words excluding allcode words not complying with said predetermined criterion and, in theevent that a coding state of the second type has been established by thepreceding code word, excluding all code words having a value of thepredetermined parameter not corresponding to the established codingstate of the second type.
 2. Method as claimed in claim 1, wherein theadditional criterion is controlling a low frequency content of themodulated signal.
 3. Method as claimed in claim 2, wherein a runningdigital sum value is established as a measure for the low frequencycontent, which value is determined over a portion of the modulatedsignal (7) and denotes for this portion the current value of adifference between the number of bit cells having a first signal valueand the number of bit cells having a second signal value, while saidselection of the code word is made so as to constrain the digital sumvalue.
 4. Method as claimed in claim 1, wherein the value of thepredetermined parameter is the logical value of p predetermined bits. 5.Method as claimed in claim 4, wherein the p predetermined bits are thefirst and thirteenth bit position.
 6. Method as claimed in claim 1,wherein the additional criterion is encoding further information. 7.Method as claimed in claim 1, wherein the modulated signal satisfies asthe predetermined criterion that each number of successive bit cellshaving a same signal value is at least d+1 and at most k+1.
 8. Method asclaimed in claim 7, wherein d is equal to 2 and k is equal to
 10. 9.Method as claimed in claim 1 wherein m is equal to 8 and n is equal to16.
 10. Method as claimed in claim 1, wherein a record carrier (120) isproduced by providing a substrate with an information pattern (123,124)representing the modulated signal (7).
 11. Coding device (140)comprising an m-to-n bit converter (60) for converting the m-bitinformation words to n-bit code words, and means (66,68) for modulatingthe n-bit code words to a modulated signal, the modulated signalsatisfying a predetermined criterion, and state establishing means (64)for establishing, on the delivery of a code word by the converter, acoding state of a first type (S1) for each of the delivered code wordsbelonging to a group (G11,G12) of a first type determined by theassociated group, and one of r coding states (S2,S3) of a second typefor each of the delivered code words belonging to a group (G2) of thesecond type determined by the associated group and by the informationword associated to the delivered code word, in which the group of thesecond type comprises at least one code word being associated with atmost r information words among which the respective information word isdistinguishable by evaluating a predetermined parameter of the followingcode word, characterized in that the m-to-n-bit converter (60) comprisesmeans for providing for each information word an associated subset ofcode words comprising at least r second type of code words mutuallydiffering in a predetermined way, and means for selecting a code wordbased on an additional criterion from the subset of code words excludingall code words not complying with said predetermined criterion and, inthe event that a coding state of the second type has been established bythe preceding code word, excluding all code words having a value of thepredetermined parameter not corresponding to the established codingstate of the second type.
 12. Coding device as claimed in claim 11,wherein the device comprises means (141,142) for recording informationon a record carrier (143) by recording an information patternrepresenting the modulated signal.
 13. Signal comprising a sequence ofsuccessive signal portions (8) each corresponding to an informationword, the signal satisfying a predetermined criterion, in which signaleach of the signal portions (8) comprises n bit cells having a first orsecond signal value, a signal portion belonging to a group (G11,G12) ofa first type of signal portions uniquely representing an informationword, and a signal portion belonging to a group (G2) of a second type ofsignal portions in combination with a successive signal portionrepresenting a unique information word, at least one signal portion ofthe group of the second type being associated with at most r informationwords among which the respective information word is distinguishable byevaluating a predetermined parameter of the successive signal portion,characterized in that the signal comprises at least one signal portionselected based on an additional criterion from a subset of signalportions excluding all signal portions not complying with saidpredetermined criterion and, in the event that the preceding signalportion belongs to the group of the second type, excluding all signalportions having a value of the predetermined parameter not beingassociated with the respective information word, and the subset beingassociated to an information word and comprising at least r signalportions mutually differing in the value of the predetermined parameter.14. Signal as claimed in claim 13, wherein the additional criterion is alow frequency content of the modulated signal.
 15. Signal as claimed inclaim 13, wherein the predetermined parameter is the logical value of ppredetermined bits.
 16. Record carrier (120) on which a signal (7) asclaimed in claim 13 is provided in a track (121) in which informationpatterns (123,124) represent the signal portions (8), which informationpatterns comprise first and second parts (123,124) alternating in thedirection of the track, the first parts present detectable firstproperties and the second parts present second propertiesdistinguishable from the first properties, and the parts having thefirst properties represent bit cells having the first signal value andthe parts having the second properties represent the bit cells havingthe second signal value.
 17. Rendering device comprising means forreading information from a track (9) on a record carrier, which devicecomprises means for scanning the track and demodulation means forretrieving code words from signal portions (8) of a signal provided inthe track, the signal satisfying a predetermined criterion, in whichsignal each of the signal portions (8) comprises n bit cells having afirst or second signal value, a signal portion belonging to a group(G11,G12) of a first type of signal portions uniquely representing aninformation word, and a signal portion belonging to a group (G2) of asecond type of signal portions in combination with a successive signalportion representing a unique information word, and a converter forconverting the code words into information words, the convertercomprising means for distinguishing the respective information word byevaluating a predetermined parameter of the following code word,characterized in that the converter comprises means for converting acode word selected from a subset of code words, the subset beingassociated to the information word and comprising at least r code wordsmutually differing in the value of the predetermined parameter. 18.Device as claimed in claim 17, wherein the converter comprises means fordetecting an additional criterion from the selection of the code wordfrom the subset.